This invention relates to the art of inspection and sorting or classifying of articles and has particular relationship to inspection and sorting or classifying of fuel pellets for nuclear reactors. While this application and it claims confine themselves to the inspection and sorting or classifying of fuel pellets, it is to be understood that to the extent that this invention is applicable to the processing of other articles than fuel pellets, such processing is within the scope of this invention. Of particular interest in this invention is the processing of plutonium-containing fuel pellets. Plutonium is a hazardous element to which people cannot safely be directly exposed. Plutonium-containing fuel pellets must be processed completely automatically in a sealed containment. It is also possible to process plutonium pellets manually in a glove box but this has proven unsatisfactory because of low production rate and excessive radiation exposure. It is an object of this invention to provide apparatus for effectively processing plutonium pellets automatically.
Typical required dimensions of a pellet in inches are: diameter 0.1945.+-.0.0002, length 0.2425.+-.0.0020, mass 1 gram, and specific gravity 10.22. It is necessary that the fuel pellets be inspected for diameter, length, flaws, and mass. In the use of the fuel pellets in a nuclear reactor, the pellets are stacked in cladding in the form of a tube which, like the pellets, is usually of circular cross-section. For effective heat transfer the pellets must be packed so that the distance between the outer surface of each pellet and the surface of the cladding is small. It is necessary that the diameter of the pellets be monitored closely to assure that the diameter of each pellet is not so large that it cannot be inserted in a tube or so small as to militate against effective heat transfer. It is necessary that the length of the pellets be monitored closely to determine the ratio of mass to length to assure proper density for adequate fissile distribution. If the pellets are properly stacked uniform fissile distribution along the length of the reactor in which the pellets are burned is achievable. It is necessary that the mass be measured to assure that the density is of the appropriate magnitude. Low density reduces heat generation; variation in density results in non-uniform fissile distribution. A measure of quality is the so-called "smear density" which is the mass divided by the length. Mass is measured to .+-.5 milligrams. Flaws in the surface of a pellet affect its heat transfer properties. Because the transfer of heat to the cladding from a flaw is impeded, hot spots are produced at flaws. In addition there is a tendency for the pellet to produce scratches in the cladding at the boundary flaws. Flaws may be classified as fissures or cracks and chips. Chips may be generally circular or generally rectangular or other shapes. In the evaluation of flaws, a flaw-quality index is assigned to each flaw. These indexes for each pellet are summed to form a surface-flaw-quality index. If this index exceeds a predetermined magnitude, the pellet is not acceptable.
As a practical matter the inspection and sorting must take place at a high rate. A test reactor having an output of 400 megawatts thermal energy requires about 2,357,000 pellets. A power station requires a reactor having an output of 1200 megawatts thermal energy. Such a reactor demands about 10,000,000 pellets. It is an object of this invention is to provide fully automatic apparatus for monitoring fuel pellets at a high rate to assure that their dimensions are within the above-stated or equivalent limits and that their surface flaws have dimensions within permissible limits and for sorting or classifying the pellets in accordance with the results of this monitoring.
Typical of the prior art are Jones' U.S. Pat. Nos. 3,221,152; 3,259,820; and 3,324,961. These patents disclose apparatus for inspecting fuel pellets for diameter, length and weight but not for flaws. The monitoring is not within the tolerance limits described above. The length and diameter are gauged mechanically by contact of the pellet by plates as in the use of calipers. The gauging cannot take place at high speed.
It is an object of this invention to overcome the drawbacks and disadvantages of the prior art and to provide apparatus for monitoring or inspecting fuel pellets for a nuclear reactor for flaws as well as dimensions and weight at a high rate and with the accuracy described above.